Radiometric dating

Ale's Stones at Kåseberga, around ten kilometres south east of Ystad, Sweden were dated at 600 AD using the Carbon–14 method on organic material found at the site.[1]

Radiometric dating (often called radioactive dating) is a way to find out how old something is. The method compares the amount of a naturally occurring radioactiveisotope and its decay products, in samples. The method uses known decay rates.[2]

It is the main way to learn the age of rocks and other geological features, including the age of the Earth itself. It may be used to date a wide range of natural and man-made materials. Fossils may be dated by taking samples of rocks from above and below the fossil's original position. Radiometric dating is also used to date archaeological materials, including ancient artifacts.

Samarium-neodymium (Sm/Nd) isochron plot of samples.[4] from the Great Dyke, Zimbabwe. The age is calculated from the slope of the isochron (line) and the original composition from the intercept of the isochron with the y-axis.

The mathematical expression that relates radioactive decay to geologic time is:[5][6]

D = D0 + N(eλt − 1)

where

t is age of the sample,

D is number of atoms of the daughter isotope in the sample,

D0 is number of atoms of the daughter isotope in the original composition,

N is number of atoms of the parent isotope in the sample, and

λ is the decay constant of the parent isotope, equal to the inverse of the radioactive half-life of the parent isotope times the natural logarithm of 2.[7]

This equation uses information on the parent and daughter isotopes at the time the material solidified. This is well known for most isotopic systems.[8][9] Plotting an isochron (straight-line graph) is used to solve the age equation graphically. It shows the age of the sample, and the original composition.

The method works best if neither the parent nuclide nor the daughter product enters or leaves the material after its formation. Anything which changes the relative amounts of the two isotopes (original and daughter) must be noted, and avoided if possible. Contamination from outside, or the loss of isotopes at any time from the rock's original formation, would change the result. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration.[10]

Measurements should be taken on samples from different parts of the rock body. This helps to counter the effects of heating and squeezing, which a rock may experience in its long history. Different dating methods may be needed to confirm the age of a sample. For example, a study of the Amitsoq gneisses from western Greenland used five different radiometric dating methods to examine twelve samples and got agreement to within 30 million years on an age of 3,640my.[11][12]